Browse Prior Art Database

Async Workstation Subsystem Capacity Enhancement Via Port Arbitration

IP.com Disclosure Number: IPCOM000121233D
Original Publication Date: 1991-Aug-01
Included in the Prior Art Database: 2005-Apr-03
Document File: 6 page(s) / 171K

Publishing Venue

IBM

Related People

Carissimo, DJ: AUTHOR [+2]

Abstract

This article describes a method to expand the maximum capability of an async workstation subsystem (measured in number of ports and line speed per attachment) via a combination of port arbitration and flow control.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 49% of the total text.

Async Workstation Subsystem Capacity Enhancement Via Port Arbitration

      This article describes a method to expand the maximum
capability of an async workstation subsystem (measured in number of
ports and line speed per attachment) via a combination of port
arbitration and flow control.

      Entry System/370* (S/370) systems utilize low-cost ASCII
displays for user terminals.  They are attached to the system via an
intelli gent adapter (ARTIC - A Real-Time Interface Co-processor)
that is plugged into the MICRO CHANNEL* of an Entry S/370 I/O
Subsystem.  This is shown in Fig. 1.

      The ARTIC adapter has an on-board 80186 microprocessor with 8
independent full duplex communication ports.  An individual port can
run at various baud rates up to 38.4 Kbps.  Data movement between the
80186 and the attached device occurs through Zilog Z8030 USARTs.  In
order to maintain the integrity of the data flow, these USARTs must
be serviced by the 80186 within critical time windows equal to a
character time (on the average).  As can be seen in Fig. 2, the
number of terminals that can be supported depends upon the fastest
character time and the interrupt handler duration.

      Realistically, all ports cannot be run at 38.4 Kbps at the same
time due to the processing overhead of character interrupts (TRANSMIT
and RECEIVE) and other functions (timers, task switching, task level
(TL) processing, etc.) that need to be performed by the 80186.  (The
actual limit in the Entry S/370 subsystem is slightly better than 3
displays operating at speeds of 19.2 Kbps.)  A compromise must,
therefore, be made between number of ports utilized on the card and
the line speed of each attachment.  When lower line speeds are used,
users are subjected to extended response times that are essentially
deemed noncompetitive due to the 'perceived' slow response time.
Likewise, when fewer ports are utilized on a card, there is a
significant cost impact to customers as well as a reduced expansion
capability as more slots are used for additional ARTIC adapters.

      The terminals are run in BLOCK mode.  In this style of
operation, the terminal head keeps the screen image buffer, and all
keystroke activity is handled locally.  The system is only notified
when an AID (Attention IDentifier) key is struck.  The system then
sends out Escape sequences to read the screen similar to a 370 Read
Modify Channel Command Word (CCW).  The terminal responds with the
modified data stream that is eventually passed to the S/370 Processor
by the ARTIC adapter and the I/O processor.  This is shown in Fig. 3.

      The problem with this approach is that substantial amounts of
data are sent inbound in bursts, causing a string of concentrated
character interrupts.

      This article describes a way in which ALL ports can support
ASCII Displays run at either 19.2 Kbps or 38.4 Kbps, hereafter
referred to as ARBITRATION.  A technique has been devised for a
cent...